Initialize \pbo for each event

Regression in commit 1f5eb5eb.
Fixes issue #74.
Update version info and changelog
Update README
Fixed headers in profile and test
Fix premature parser exit on }{ in drawing mode

Fixes #75.
Implement fast quad-tree rasterizer in C and x86/SSE2/AVX2

Signed-off-by: Rodger Combs <rodger.combs@gmail.com>
    style overrides: subtitle styling override

    This adds a mechanism that attempts to override dialogue lines, but not
    anything else, like typesetting.

    Requires the libass API user to request this explicitly. If this is not
    requested, libass will (or should) behave like without this commit.

    mpv patch included as 0001-sub-add-ass-style-override-force-option.patch
    (read the commit message)